SSS#002 Hum after Delay

[award70]

Hi Everyone,

I recently built SSS#002 using Erwin's boards and a 5 piece chassis (a la ijedouglas). After powering on and warming up everything works and sounds good until about 10-15 seconds after warmup. After the 10-15 second delay a ~120Hz hum develops. It's not super loud but I don't think it should be there. I've re-flowed the power supply capacitor solder joints as well as most of the other solder points and they seem good. Any idea what might be causing the delayed hum? Could it take that long for the can capacitors to fully charge?

The schematic I'm using is the "SSS#002 REDRAW DRAFT SCHEMATIC" with a couple changes. I added a 60VAC toroidal transformer and used Erwin's bi-polar power supply board for the cf driver stage. For power tube bias I used the normal 60v tap from the power transformer. I ran the heater wires above the boards and added a hum-dinger. With the hum-dinger I can dial out all of the heater hum/noise.

The amp is playable the way it is but it can't be normal to have that delayed hum. Picture is attached (sorry not as pretty as most builds ).

Any thoughts, ideas are appreciated.


Alan

[martin manning]

I added a 60VAC toroidal transformer and used Erwin's bi-polar power supply board for the cf driver stage. For power tube bias I used the normal 60v tap from the power transformer.

The bipolar supply is intended to be run from the bias tap, but you can certainly run it from your 60VAC toroidal. The bias tap would then be unused, and there is no power tube bias other than the cathode voltages on the driver tubes.

[award70]

Sorry you are correct. The bias is applied to the grids of the cathode follower drivers (referred to as "V BIAS" in the schematic). I am using the normal power transformer bias tap to supply that bias rather than tapping off the bipolar supply. It should still work correctly this way shouldn't it?

[bepone]

What means 10-15 sec? After warmup?

Warmup is usually after one minute, heaters on, high voltage on. 60 or 60+15sec =75 seconds is the same.

120 Hz humm is coming from the rectifier, and means some triode stage receiving on the anode non filtered supply and is inserting it in the circuit. And delayed humm means this happening when this triode is starting to heat up. Which one, should be inspected by scope. Probably REVERB SEND triode, but is just a blind guess

[award70]

What means 10-15 sec? After warmup?

Warmup is usually after one minute, heaters on, high voltage on. 60 or 60+15sec =75 seconds is the same.

120 Hz humm is coming from the rectifier, and means some triode stage receiving on the anode non filtered supply and is inserting it in the circuit. And delayed humm means this happening when this triode is starting to heat up. Which one, should be inspected by scope. Probably REVERB SEND triode, but is just a blind gues

By "warmup" I mean the amp has been on long enough to produce sound. I don't mean the full 75 seconds or so. I mean Power-On, stand-by off, can hear sound/play guitar once the valves are warmed up around 15 seconds , everything good. Around 10 seconds after that, the hum starts.

I'll check the reverb send triode.

[bepone]

Usually ecc83s are warmed up in less than minute, if something happening after, means worn out tube (slow start) then replace this tube.

But this will not resolve your problem, you need better filtering, so add more RC (several kohms+22-47uF/500V cap) to the stage where is happening

[award70]

But this will not resolve your problem, you need better filtering, so add more RC (several kohms+22-47uF/500V cap) to the stage where is happening

Ok I'll focus on the power supply and also try to determine which stage is causing the hum.


Thanks.

[erwin_ve]

Hi Alan!,

Looking at your build pics I see your filaments are floating. Are they close to any capacitors? I cant judge from the pics, it could be a source of hum.

Erwin

[award70]

Looking at your build pics I see your filaments are floating. Are they close to any capacitors? I cant judge from the pics, it could be a source of hum.

Hi Erwin,
The filament wiring does get close to some of the caps. I have chopsticked around the heater wiring and don’t hear any changes when moving the wiring. There is hum from the heaters but I am able to balance it with the hum dinger so that it is basically gone.


Alan

[martin manning]

I suggest you do your driver bias like this:

[award70]

I suggest you do your driver bias like this

I will give that a shot Martin. Thanks

[martin manning]

I think your PCB is basically set up that way.

[award70]

I made the changes to the bias circuit and that did the trick. The hum is gone! Thank you Mr Manning.

[ViperDoc]

I've noted Martin's bias voltages and will put them on an amended version of the schematic. That SSS002 is a "DRAFT"!!!, but it's great to hear your amp is up and running. I want to make sure I drew the Manning/Colgan bias circuit correctly. Any other voltage recommendations you all have, please send them my way so I can add them.

[martin manning]

I made the changes to the bias circuit and that did the trick. The hum is gone! Thank you Mr Manning.

Sure, glad it's fixed! Are you running the drivers on the 60VAC toroidal or the 60V bias tap now?
If you find you don't like the uneven steps in the filters, consider updating the 3n cap location and the other resistor and cap values to the ones I have proposed.

I've noted Martin's bias voltages and will put them on an amended version of the schematic. That SSS002 is a "DRAFT"!!!, but it's great to hear your amp is up and running. I want to make sure I drew the Manning/Colgan bias circuit correctly. Any other voltage recommendations you all have, please send them my way so I can add them.

No secrets there. Like any CF, the cathode voltage will rise until it's a volt or two above the grid, so the grid bias voltage has to be set a volt or two below the desired power tube grid voltage.